📝 added exception 403 to documentation of at (#888)

The at function throws json::out_of_range.403 when a nonexistent object key is provided (just like the usual at function). This was not documented and users could assume json::out_of_range.404 would be thrown instead.

- Updated documentation.
- Added example code.

doc/examples/at_json_pointer_const.cpp

@@ -55,6 +55,17 @@ int main()
         std::cout << e.what() << '\n';
     }
 
+    // out_of_range.403
+    try
+    {
+        // try to use a JSON pointer to an nonexistent object key
+        json::const_reference ref = j.at("/foo"_json_pointer);
+    }
+    catch (json::out_of_range& e)
+    {
+        std::cout << e.what() << '\n';
+    }
+
     // out_of_range.404
     try
     {